Serenity Now comes from Seinfeld, where Frank Costanza, played by Jerry Stiller, adopts the phrase as a shouted anger-management mantra in the 1997 episode of the same name. Instead of calming him, the yelling becomes the joke, and the line stuck because it captures the gap between trying to stay peaceful and actually losing your temper. It later spread through sitcom clips, reaction videos and soundboards, where creators still use it to punctuate stress, irony and over-the-top frustration.